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Clojure: JVM and errors

A bit about the JVM platform that Clojure uses. Very often, the guts of the JVM early stage look out, which makes most runtime errors hard to read, especially if the error occurred somewhere in the depths. For example:

;call (1)
Execution error (ClassCastException) at user/eval1 (REPL:1).
class java.lang.Long cannot be cast to class clojure.lang.IFn (java.lang.Long is in module java.base of loader 'bootstrap'; clojure.lang.IFn is in unnamed module of loader 'app')

The error does not look very informative...

Consider the most common errors that can occur and understand what they mean:

X can not be cast to Y

Suppose we have some type X, but the function being called expects its argument to be of type Y. Clojure tries to cast type X to type Y, but crashes with an error.

; Let's call the number as a function
(def x 10)
Execution error (ClassCastException) at user/eval138 (REPL:1).
class java.lang.Long cannot be cast to class clojure.lang.IFn

Don't know how to create Y: from X

The error is very similar to the example above, Clojure tries to convert X to Y but cannot.

; Let's try to iterate simple number, instead of collection
(reduce (fn [x y] (into x (+ x 1))) [1 2 3])
Execution error (IllegalArgumentException) at user/eval1$fn (REPL:1).
Don't know how to create ISeq from: java.lang.Long

Wrong number of args (X) passed to: Y

Here everything is clear from the name, we pass the wrong number of arguments to the function:

; Function first takes only one argument
(first 1 2 3)
Execution error (ArityException) at user/eval3 (REPL:1).
Wrong number of args (3) passed to: clojure.core/first--5402

X - failed: even-number-of-forms? at: [:bindings] spec: :clojure.core.specs.alpha/bindings

Such an error occurs during the compilation of Clojure code, for example we have a form let that uses a vector to connect identifier and data, inside which there are two elements, the identifier itself and the data. Clojure internally uses clojure.spec to check such declarations, so the error message may be slightly different each time:

; valid code
(let [a 2]
  (+ a 4)) ; => 6

; incorrect code
(let [a b 3]
  (+ a b))
Syntax error macroexpanding clojure.core/let at (REPL:1:1).
[a b 3] - failed: even-number-of-forms? at: [:bindings] spec: :clojure.core.specs.alpha/bindings

Although the error output may scare and discourage newcomers at first, in general it is possible to get used to this error output. However, thanks to the JVM, Clojure has all the features of the Java world and more!

(Integer/parseInt "123") ; 123

In the code above, we called a static Java method that parses a string into a number. The same applies to libraries written for Java. There are also projects that use Java, Scala and Clojure at the same time.


Print to the standard output stream (using the println function) the result of parsing the string 256 (using the Integer/parseInt method).

The exercise doesn't pass checking. What to do? 😶

If you've reached a deadlock it's time to ask your question in the «Discussions». How ask a question correctly:

  • Be sure to attach the test output, without it it's almost impossible to figure out what went wrong, even if you show your code. It's complicated for developers to execute code in their heads, but having a mistake before their eyes most probably will be helpful.
In my environment the code works, but not here 🤨

Tests are designed so that they test the solution in different ways and against different data. Often the solution works with one kind of input data but doesn't work with others. Check the «Tests» tab to figure this out, you can find hints at the error output.

My code is different from the teacher's one 🤔

It's fine. 🙆 One task in programming can be solved in many different ways. If your code passed all tests, it complies with the task conditions.

In some rare cases, the solution may be adjusted to the tests, but this can be seen immediately.

I've read the lessons but nothing is clear 🙄

It's hard to make educational materials that will suit everyone. We do our best but there is always something to improve. If you see a material that is not clear to you, describe the problem in “Discussions”. It will be great if you'll write unclear points in the question form. Usually, we need a few days for corrections.

By the way, you can participate in courses improvement. There is a link below to the lessons course code which you can edit right in your browser.


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